The present invention is directed in general toward phase-locked loops and, more particularly, toward a digital phase locked loop implemented with programmable digital signal processing methods.
There are many devices and methods presently available for determining frequency and for locking to an analog signal. Generally, they fall into two broad classes, to wit, analog and digital. Although analog phase-locked loops once dominated the art, continued progress in semiconductor technology, enhancing the performance, speed, and reliability of integrated circuits while simultaneously reducing their size and cost, has resulted in strong interest in the implementation of the phase-locked loop in the digital domain.
Aside from the obvious size and cost advantages associated with digital systems, a digital version of the phase locked loop alleviates many other problems associated with its analog counterpart, namely: sensitivity to dc drifts and component saturations, difficulties encountered in building higher order loops, and the need for initial calibration and periodic adjustments. In addition, since many systems requiring phase locking already employ digital signal processing devices to perform other functions, a digital phase locked loop can be readily implemented to make use of these processing devices thereby eliminating the need for other specialized circuitry and further reducing circuit cost.
Despite the increasing demand for digital phase locked loops, present day digital implementations of phase locking devices have proven inadequate for many applications. This is because conventional digital devices for locking a local signal to an analog input signal determine the frequency of the input signal by measuring the time interval between its zero crossings. However, this method of measuring the input frequency is sensitive to noise which may cause the timing of the zero crossing to fluctuate and, therefore, these systems have proven inaccurate. Further, this method is costly to implement because a digital counter must be employed and dedicated to measure the time interval between zero crossings. Alternatively, a processor may be used to measure this time interval, however, the processor performing the measurement must either be dedicated to the measurement or must be interrupted in the performance of other tasks to measure the zero crossing time.
There exists, therefore, a need to determine the frequency of an analog signal with a high degree of accuracy and to use this information to improve both the performance and productivity of signal processing elements used in digital phase locked loops.